Romex power cable as speaker cables?

Solid core wire is usually a good conductor for interconnects and thick gauge is great for speaker cable. The less the stray strands, the less they can interfere with the signal transmission.
But, Romex wire uses the cheapest form of copper. The concept is good but the quality is just not there. IMHO.

I actually made (2) 12 foot lengths of pure soft annealed 9999 12 gauge silver wire and it sounds great. But this was before the silver prices got crazy expensive.

ozzy

I know of at least 1 mega-bucks speaker company that uses solid core, electrical wire for their internal wiring. 

Saves on materials, but adds significantly to the time needed for assembly. 

I don’t use it for my main, but because I had to carry the signal 60 feet from my system to my in ceiling kitchen speakers, I figured it was worth trying 10 gauge NM wire. The sound is a little bright but I think that’s more the speakers, and the fact that they’re in ceiling, than the wire. 
That said, later on, when I Upgraded my main speakers, I bi-wired them with good AQ cables. 

The copper part is fine but the dielectric is a bit of a problem because of dielectric absorption, which is increasingly problematic with frequency. You really want something like teflon or at least a good polyester.

I run ESL's which require a series input resistor, so I use open-to-the-air nichrome wire as speaker cable which tunes the system at the same time. The result is, you might say, no speaker cables at all! 

I myself need long speaker cables as my audio equpment is situated to my right side.  I like all my equipment within easy reach, thus necessitating the need for long speaker cables.  A few years ago by chanch I had obtained a one foot length of Shunyata's top of the line speaker cables.  I also had lots of Jenna Lab's 18 gage hook up wires.  Again, by chanch I hooked the one foot Shunyata speaker cables to my speakers and a single length of the Jenna Lab's 18 gage hook up wires between the Shunyata wires and my amplifier at the time-a Rega Orisis integrated amplfier.  The results were spectacular.  Over the years I changed the Shunyata Speaker wires with MG Audio Design's tp of the line, one foot pair of their speaker wires.  The results were far superior to the now7-8 year old Shunyata speaker cables.  Over time I aded first a second Jenna Lab's 18 gage hook up wires to the combination-now 8 of them in total-the results were again improved greatly.  Finally I added a third run of the Jenna Lab's wires to my present "speaker wire" combination with simply incredible results.  By the way I first learned about the MG Audo Design wires from the late Arnie Nudell as they were his personal choices.  Later I learned that Arnie's good friend, Paul McGowan also uses them.  Anyway, presently my "speaker Wires" average to a 14.5 foot pair of speaker wires, which today would cost about $1800.  The result is a pair of speaker wires that will compete with the "Big Boys" stuff.  In actuality the eventual 12 wire runs of the Jenna Lab's 18 gaage hook up wires allow the one foot pair if the MG Audio Design speaker wires to d their stuff.  The long runs of my Jena Lab's 18 gage hook up wires-12 in all-are so "pure" in their ability to allow my one foot pair of MG Audio Design wires to pass thru them.  Thus if instead I was just using a long pair of the MG Audio Design wires by themself, that the audio differences would most likely would be not noticeable.  Anyway my "speaker wire" solution works.  It is as simple to create as possible-just a wire stripper and a lot of Jenna Lab's hook up wires to strip.  Just takes a few minutes to create and put together.  But again, they can and do create a truly spectacular pair of speaker wires for a fraction of the cost off the competition.  I myself was only fortunate to, again by chance, to combine two different wire products by two small wire manufcturers to achieve something truly wonderful.  I have no financial interest whatsoever with both companies.  I just use their products.